tests: fix broken &&-chains in compound statements

The top-level &&-chain checker built into t/test-lib.sh causes tests to
magically exit with code 117 if the &&-chain is broken. However, it has
the shortcoming that the magic does not work within `{...}` groups,
`(...)` subshells, `$(...)` substitutions, or within bodies of compound
statements, such as `if`, `for`, `while`, `case`, etc. `chainlint.sed`
partly fills in the gap by catching broken &&-chains in `(...)`
subshells, but bugs can still lurk behind broken &&-chains in the other
cases.

Fix broken &&-chains in compound statements in order to reduce the
number of possible lurking bugs.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/p5302-pack-index.sh

@@ -21,7 +21,7 @@ test_expect_success 'set up thread-counting tests' '
 	threads= &&
 	while test $t -gt 0
 	do
-		threads="$t $threads"
+		threads="$t $threads" &&
 		t=$((t / 2))
 	done
 '